Protective silicate coating



2,952,562 Patented Sept. 13, 1960 2,952,562 PROTECTIVE SILICATE COATINGLester Morris, Whittier, and Flora Lombardo, Lynwo'od, Calif., assignorsto Amercoat Corporation, South Gate, Calif., a corporation of CaliforniaNo Drawing. Filed Jan. 27, 1958, Ser'. No. 711,126 6 Claims. (Cl.117-'62) This invention relates to protective coatings of the silicatetype, and particularly to methods and materials for curing suchcoatings.

This application is a continuation in part of our copending applicationSer. No. 376,260, filed August 24, 1953, now abandoned, which is, inturn, a continuation in part of our application Ser. No. 296,244, filedJune 28, 1952, now abandoned.

Protective coatings comprising the reaction products of alkali silicateswith metals, such as zinc and lead and/ or compounds thereof, have beenused more or less extensively for protecting iron, steel and othersurfaces against weathering and corrosion. In the use of such coatings,it has been common practice to subject the applied coating materials toheat treatment, preferably a baking treatment at a temperature of fromabout 200 F. to about 500 F. for sufficient time to cause the coating tobecome hard, impermeable and insoluble. Relatively small parts, whichmay be coated and subsequently subjected to heat treatment at thefactory, may be effectively protected by such coatings. However, in thecase of large structures fabricated in the field, as for example storagetanks, towers, structural steel frameworks and outdoor construction ofvarious types, such heat treating of the coated surfaces isimpracticable. Air drying under ordinary conditions fails to effect therequired hardening and insolubilizing of the coating; and wherestructural parts must be shaped during installation, pretreatment ofindividual sections is not practicable because a previously appliedcoating becomes damaged by the bending and shaping incident toinstallation.

It is among the objects of the present invention to provide methods andmaterials for treating silicate coatings to cure the same to an adequatehardness, impermeability and insolubility after fabrication in thefield, and without requiring heat treatment.

It is also an object of the invention to provide a curing agent whichmay be applied to a fihn of silicate coating material, and which willreact therewith to produce a hard and insoluble surface coating foreffectively protecting the coated surface from corrosion and otherdamaging reactions.

It is a further object to provide coating and curing materials which maybe compounded in quantity for storage and shipment, and which may bereadily applied by spraying or brushing on field installations, or bydipping prefabricated parts at the factory.

A still further object is to provide an agent which will quickly andeffectively cure silicate coating materials, and produce a surface ofbright and attractive appearance over which other coatings of the sameor different character may be effectively applied.

The present invention contemplates the treatment of a film of coatingmaterial of the type of zinc silicate, lead silicate, and zinc-leadsilicate when applied to a base or substrate, with a liquid curing agentcomprising a solution of a salt capable of yielding an acid radical andwhich will react with the materials of the silicate film to effecthardening and insolubilization of the same.

While the curing agent may include as a solvent an organicwater-miscible liquid, or water, or both, the amount of water presentmay be advantageously limited to an amount sufiicient to cause formationof an acid reaction by hydrolysis of the dissolved salt, but notsufficient to cause impairment of the surface of the coating to whichthe curing liquid is applied.

In preparing the liquid curing agent of the present invention, the acidreacting salt may be either an inorganic or organic salt of a strongmineral acid, such as, for example, acid reacting salts of phosphoric,hydrochloric, nitric, sulfuric or chrorrric acids, or there may be anorganic amine salt of such acids. Whichever type is used, the acid saltmust be soluble in the organic or aqueous solvent employed, and must becapable upon hydrolysis of liberating an acid radical which will reactwith a silicate film of the character described to produce a hardinsoluble coating. Care must be taken to avoid acidic salts which byhydrolysis release their acid radicals so rapidly, and in aconcentration so great, as to cause stratification in the film beforethe curing effect is complete, or in which the pressure of excess watermaterially affects the film before the curing action is complete.

The vehicle in which the acid salt is dissolved may comprise eitherwater or an organic water miscible compound in combination with alimited amount of water, such as, for example, a monohydric orpolyhydric alcohol, ether, or ketone. There may be advantageouslyemployed for this purpose, or example, aliphatic alcohols, such asmethyl, ethyl, propyl, isopropyl, butyl, and higher alcohols. Whileisopropyl alcohol is preferred on account of cost and superior solventproperties, the other alcohols, may be used as well. Methyl alcohol,however, presents problems of toxicity, while ethyl alcohol involveshigher costs, presence of denaturants, and other practicaldisadvantages. Glycols such as ethylene glycol or propylene glycol, andglycerol may be used. Water soluble others such as Carbitol, and dioxanehave been found suitable. Water soluble ketones such as acetone andmethyl ethyl ketone may be substituted with satisfactory results.

In a preferred embodiment of the present invention, the liquid curingagent comprises an organic vehicle such as an alcohol, containing onlyenough water to dissolve and aid the hydrolysis of the acid salt used.Such acid salt must be capable of yielding an acid radical which willreact with the ingredients of a silicate film of the character of zincsilicate, lead silicate, and zinc-lead sili- Cate, as for example thecoating known commercially as Dimetcote and similar coatings produced bythe reaction of alkali silicates with metals and/or metallic compounds.In said preferred embodiment, isopropyl alcohol is advantageouslyemployed in admixture with a small quantity of water as the organicvehicle. A salt which has been found to be especially well suited to thepurposes of the present invention is dibutylamine phosphate, produced bythe reaction of di-n-butylamine with phosphoric acid. The active factorin this product is phosphoric acid, the acid radical of which willcombine with ingredients of the silicate film to render the film hardand insoluble. Phosphoric acid, used alone, reacts with the zinc-leadsilicate film too violently, with the result that the film is caused tocrack and peel. When combined with an amine, however, the phosphoricacid radical is freed relatively slowly and in relatively diluteconcentration by hydrolysis, so that its reaction with the silicate filmis correspondingly retarded. It is believed that, if the reaction withthe silicate film is too rapid, a differential shrinkage occurs withinthe film, and causes the surface to crack and/or peel before theunderlying material fully reacts. When released slowly by hydrolysis,the acid radical appears to react with the film mate rial substantiallyuniformly throughout its depth and area, probably because the solutionhas time to penetrate through the film and react with the underlyingstrata before the surface is hardened and rendered insoluble.

While the chemical reaction is complicated, and is not fully understood,it is believed that when the curing agent is sprayed or brushed over asilicate coating, a dehydration of the coating occurs. At the same timethe dibutyl amine phosphate hydrolyzes into dibutylamine and phosphoricacid. As the acid radical is slowly released, it reacts with free alkalipresent in the silicate coating to form phosphate salts. Silicic acid isreleased from the alkali silicate of the coating, and the zinc and leadcomponent is activated by the dibutylamine. The zinc or lead then reactswith the silicic acid to form zinc or lead silicates; and some of thephosphoric'acid reacts with the lead or zinc to produce phosphates. Atthe same time, the released phosphoric acid serves to passivate the zincor lead on the surface as well as any iron that may be present orexposed. As the dibutylamine phosphate hydrolyzes, the dibutylaminetends to volatilize, and thereby shift the equilibrium to releaseadditional phosphoric acid for the cure.

In preparing the curing agent, the phosphoric acid and dibutylamine arepreferably mixed in substantially molecular ratio to obtain maximumproduction of the dibutylamine phosphate in the solution. However, theamounts may be varied through a considerable range. Thus, phosphoricacid alone, without any of the amine, will react with the silicatecoating; but will result in a cracking and/ or peeling of the coatingsuch as to render the coating unacceptable, and hence some amine must bepresent to modify the action. A slight excess of phosphoric acid ispermissible so long as it does not unduly speed the reaction so as tocause cracking or peeling. An excess of the amine is not objectionableup to a ratio of about 1 part of acid to 2 parts of amine; but furtherexcess amine increases cost with no beneficial result.

The presence of some water in excess of that avail able by dehydrationof the silicate fihn is necessary to hold an adequate amount of theamine and amine phosphate in solution, and to aid the hydrolyzation ofthe salt. Excess water tends to react with and aiiect the originalsilicate film materials. A ratio of about 95 parts of alcohol to 5 partsof water is preferred, but the amount of water may be decreasedsomewhat, if desired, and may be increased to a point where noobjectionable impairment of the final coating results. By increasing theamount of water, the amount of amine phosphate held in solution in thealcohol-water vehicle can be somewhat increased. It is desirable toincrease the concentration of amine phosphate to the maximum possiblewithout causing damage to the film by the presence of excess water. ifsome of the dissolved salt precipitates out of the solution, additionalwater may be added to dissolve it; or, preferably, the solution may beheated sufiiciently to redissolve the precipitate.

In place of the di-n-butyl amine, it is practicable to use any .minewhich will react with phosphoric acid or its equivalent to produce anamine salt soluble in an organic solvent which is miscible with waterand which will hydrolize in solution to yield an acid radical suitablefor reacting with a silicate film to insolubilize the coating. Forexample, it has been found that dipropylamine, triethyl amine,morpholine, ethyl morpholine, monoethanol amine, triethanol amine andethylene diamine may be used and will produce satisfactory results. Thesolubility in alcohol or its equivalent varies; and, where a substitutefor the preferred amine is sought, variation in the solvent vehicle toobtain a solution of the amine of adequate strength and hydrolyzingproperty may be required. For commercial purposes, the availability andcost of the various materials in a particular market may determine thespecific amine and the specific solvent most practicable for aparticular locality.

In determining the availability of any particular amine for preparingthe curing agent of the present invention, the selected amine andphosphoric acid are mixed in substantially molecular proportion toproduce a saturated solution in alcohol or, its equivalent containingonly the commercially acceptable amount of water. If, at ordinarytemperature, the salt formed by the reaction of acid and amine fails todissolve or precipitates at normal temperature, water may be added insmall increments until the salt is dissolved. Several formulas should beprepared in which the pH varies from 7 to 2, by increasing the excess ofacid. The formulas are then tested by brushing the agent over silicatefilms, and washing off the cured coating at various intervals rangingfrom /2 hour to 24- hours. If specimens partially submerged in water donot crack or peel at the water line, the cure is considered adequate,and use of the substituted, material is acceptable. Salt spray tests aredesirable if severe service conditions are contemplated. In general, anyamine which reacts with phosphoric acid to produce a phosphate saltsoluble in alcohol containing only a relatively small amount of water,may be used. The availability of any particular amine for the purpose ofthis invention may be readily determined by testing as above indicated.

For some purposes, other acids and acid salts may be substituted forphosphoric acid and the acid salts thereof above described. For example,acid salts such as magnesium chloride, ammonium phosphate, aluminumsulphate, and other acid salts of hydrochloric nitric, sulphuric, andchromic acid, which are soluble in alcohol and its equivalent and byhydrolysis liberate an acid radical which will react with a silicatefilm of the character described to produce a hard insoluble coating, maybe used. In making such substitutions, care must be taken to avoidacidic salts which by hydrolysis release their acid radicals so rapidly,and in concentration so great, as to cause stratification in the filmbefore the curing effect is complete, or in which the presence of excesswater materially afiects the film before the curing is completed.Preliminary tests along the lines above explained will readily indicatethe operability of various materials as substitutes for the preferredmaterials above noted.

In practice, the curing agent of the present invention is sprayed orbrushed over surfaces previously' coated with a silicate film of thecharacter above described which has been air dried on the surface. Thethickness of the silicate film is preferably kept between about 1.5 to2.0 mils. Films of greater thickness, in. the order of 2.0 to 3.0 mils,usually require an excess of the curing agent; and, for thicknesses ofabout 3.0 to. 5.0 mils, an additional coat of curing agent should beapplied. The curing agent is permitted to complete its reaction with thesilicate film, and any excess is then washed off.

The insoluble coating so produced is of a hardness equal to, if notexceeding, that of a baked silicate coat-- ing, and is materially lesscostly. Additional coats of a similar nature may be applied over aninitial coating, with satisfactory results. Coatings of other materialsalso may be applied, and afford good ahesion With a minimum ofblistering.

The invention may be further illustrated by the following examples:

Example I The curing agent may be prepared by mixing materials in thefollowing proportions:

The alcohol, Water and amine are placed in a mild steel tank orequivalent container, and the phosphoric acid is stirred carefully intothe mixture. The mixture tends to become hot, but ordinarly will notboil. During the addition of the acid, a white flutfy salt sometimesappears; but, upon further stirring and the addition of further acid,the salt is dissolved. After the reaction is complete, the liquidproduct which results may be stored in glass, or terne plate. Thephosphoric acid and di-nbutyl amine combine to form dibutylaminephosphate, which is dissolved in the alcohol and water.

Example II A strong aqueous solution of magnesium chloride was preparedby dissolving approximately 65 lbs. of magnesium chloride hexahydrate,MgCl -6H O, in an equal weight of water. The resulting solution containsapproximately 30 to 32 percent of MgCl When applied to a silicatecoating this solution can be used for curing but care must be taken toavoid formation of white stains or bloom.

Example III A solution of magnesium chloride in methyl alcohol wasprepared by dissolving approximately 50 lbs. of salt in approximately 15gallons of commercial methyl alcohol containing approximately of water.This solution can also be used for curing in accordance with the presentinvention.

We claim:

1. A protective coating of the character described ap plied to asubstrate and comprising: the reaction product of a dried film ofsilicate selected from the class consisting of zinc silicate, leadsilicate, zinc-lead silicate and mixtures thereof with an acid radicalreleased by hydrolysis from a substantially nonaqueous solution of anorganic amine salt of an acid selected from the group consisting ofphosphoric acid, sulfuric acid, hydrochloric acid, nitric acid andchromic acid in the presence of about 5% water.

2. A protective coating of the character described applied to asubstrate and comprising the product resulting from the reaction of anacid radical released by hydrolysis from a substantially nonaqueoussolution of an organic amine salt of an acid selected from the groupconsisting of phosphoric acid, sulfuric acid, hydrochloric acid, nitricacid and chromic acid, with a film of a silicate selected from the classconsisting of zinc silicate, lead silicate, zinclead silicate andmixtures thereof, in the presence of about 5% water.

3. A protective coating of the character described applied to asubstrate and comprising the reaction product of a phosphoric acidradical released by hydrolysis from a substantially nonaqueous solutionof an organic amine phosphate with a film of a silicate selected fromthe class consisting of zinc silicate, lead silicate, zinc-lead silicateand mixtures thereof and convertible by reaction with said phosphoricacid radical in the presence of about 5% Water to a hard insolublecoating.

4. A protective coating of the character described applied to asubstrate and comprising the reaction product of a film of silicateselected from the class consisting of zinc silicate, lead silicate,zinc-lead silicate and mixtures thereof, and a solution of dibutylaminephosphate in an organic solvent, said solution being miscible withwater, and capable of yielding an acid radical in the presence of asilicate film to produce a hard, insoluble coating the proportion ofWater to said amine salt solution being about 5 percent.

5. The method of curing a silicate coating selected from the classconsisting of zinc silicate, lead silicate, zinc-lead silicate andmixtures thereof, which comprises applying to said silicate coating analcohol solution of the reaction product of phosphoric acid and anorganic amine, said solution containing about 5% water.

6. The method of coating a metal structure which comprises: applying .tothe surface of said structure a coat of a silicate material selectedfrom the class consisting of zinc silicate, lead silicate, zinc-leadsilicate and mixtures thereof, the thickness of said coat not exceedingabout 2 mils, air drying said coat, and then applying to said coat afilm of a curing agent comprising an alcohol solution of an organic,acidic, amine salt of an acid selected from the group consisting ofphosphoric acid, sulfuric acid, hydrochloric acid, nitric acid andchromic acid subject to hydrolysis in the presence of said coat torelease an acid radical which will react with said coat to harden andinsolubilize said coat.

References Cited in the file of this patent UNITED STATES PATENTS 71,210Pelletier Nov. 19, 1867 419,657 Gesner Jan. 21, 1890 963,810 Sala July12, 1910 2,576,845 McDonald Nov. 27, 1951 2,688,598 McNeely Sept. 7,1954 2,717,843 Wachter et a1 Sept. 13, 1955

6. THE METHOD OF COATING OF A METAL STRUCTURE WHICH COMPRISES: APPLYINGTO THE SURFACE OF SAID STRUCTURE A COAT OF A SILICATE MATERIAL SELECTEDFROM THE CLASS CONSISTING OF ZINC SILICATE, LEAD SILICATE, ZINC-LEADSILICATE AND MIXTURES THEREOF, THE THICKNESS OF SAID COAT NOT EXCEEDINGABOUT 2 MILS, AIR DRYING SAID COAT, AND THEN APPLYING TO SAID COAT AFILM OF A CURING AGENT COMPRISING AN ALCOHOL SOLUTION OF AN ORGANIC,ACIDIC, AMINE SALT OF AN ACID SELECTED FROM THE GROUP CONSISTING OFPHOSPHORIC ACID, SULFURIC ACID, HYDROCHLORIC ACID, NITRIC ACID ANDCHROMIC ACID SUBJECT TO HYDROLYSIS IN THE PRESENCE OF SAID COAT TORELEASE AN ACID RADICAL WHICH WILL REACT WITH SAID COAT TO HARDEN ANDINSOLUBILIZE SAID COAT.